Pneumatic tire

ABSTRACT

A standard grounding surface shape F 0  becomes a maximum grounding length Lc at a tire equator C and becomes a shortest grounding length Lm between the tire equator C and a grounding edge TE. The shortest grounding length Lm is located in a region away from the tire equator C by a distance of 0.5 to 0.9 times of a tread half width TW. A rubber thickness t 1  between the carcass cord and the belt cord is gradually increased outward in the axial direction of the tire from the tire equator, and the rubber thickness t 1  in a half width position Q which is away from the tire equator by a distance of 0.55 times of the tread half width TW is set to 0.5 to 3.0 mm. A ratio t 2   q /t 2   c  between a tread entire thickness t 2   c  in the half width position Q and a tread entire thickness t 2   c  in the tire equator is set to 1.01±0.05.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP02/10495 which has an Internationalfiling date of Oct. 9, 2002, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to a pneumatic tire which is suitable as aheavy load tire into which a high internal pressure is charged and whichrestrains uneven wear at a tread portion.

BACKGROUND TECHNIQUE

In a heavy load tire for example, generally, its tread outline shape ais formed into a single arc shape in a vulcanizing metal mold as simplyshown in FIG. 6.

In such a tire, however, in a standard internal pressure state in whichthe tire is assembled into a rim and a standard internal pressure ischarged into the tire, there is a tendency that a tread surface swellsradially outward in a region y which is separated from a tire equator bya distance of 0.5 to 0.7 times of a tread half width. For this reason, adifference in circumferential lengths between a swelling portion b and atread grounding end e is increased, slip between the tread surface onthe side of the tread grounding end and a road surface is generated, anduneven wear such as so-called shoulder wear is prone to be generated.

On the other hand, in order to restrain the shoulder wear, JapanesePatent Application Laid-open No. H7-164823 and the like disclose atechnique in which the outline of the tread in the vulcanizing metalmold is formed into a double radius shape in which a portion of thetread on the side of the tread grounding end is an arc having a greaterradius of curvature than a portion of the tread on the side of the tireequator, thereby bringing the outline of the tread in the standardinternal pressure state closer to the single arc to restrain theshoulder portion decrease wearing.

Such a technique can restrain the shoulder portion decrease wearing insome degree but there is a problem that new uneven wear is produced inthe region y. Especially when a circumferential groove is formed in theregion y, uneven wears are seriously generated in inside and outside ofthe circumferential groove in the axial direction of the tire.

Thereupon, it is an object of the present invention to provide apneumatic tire in which the uneven wear from the tire equator to thetread edge is restrained and the wear resistance is enhanced.

DISCLOSURE OF THE INVENTION

An invention of claim 1 of this application provides a pneumatic tirecomprising a carcass including a carcass ply extending from a treadportion to a bead core of a bead portion through a sidewall portion, anda belt layer having a belt ply arranged inside the tread portion andoutside the carcass, wherein

a standard grounding surface shape when a standard load is applied tothe pneumatic tire in a state in which the tire is assembled into astandard rim and a standard internal pressure is charged into the tireincludes an inner region including a shortest grounding length Lmbetween a central region including a grounding length Lc at a tireequator in which a grounding length L in a circumferential direction ofthe tire becomes maximum and a tread edge region including a groundinglength Le at a grounding edge, said grounding length Lm the shorter thanthe grounding length Lc and the grounding length Le,

the shortest grounding length Lm is located in a range apart from thetire equator by a distance of 0.5 to 0.9 times of a tread half width TW,

a rubber thickness t1 between a carcass cord of the carcass ply and abelt cord of the radially inward belt ply which is the closest to thecarcass cord is gradually increased outward in an axial direction of thetire from the tire equator, the rubber thickness t1 q in a half widthposition of 0.55 times of the tread half width TW from the tire equatoris set to 0.5 to 3.0 mm,

a ratio t2 q/t2 c between an entire thickness t2 q of the tread portionif the half width position and an entire thickness t2 c of the treadportion in the tire equator is set to 1.01±0.05.

According to an invention of claim 2, in 80% internal pressure groundingsurface shape when a standard load is applied to the tire in 80%internal pressure state in which the tire is assembled into the standardrim and 80% internal pressure of the standard internal pressure ischarged, a ratio Lsm/Lsc between a grounding length Lsm which becomesthe shortest in a region from a position away from the tire equator by adistance of 0.5 times of the tread half width TW to the grounding edgeand a grounding length Lsc at the tire equator is set to 0.75 to 0.99.

In this specification, in specifications system including specificationson which a tire is based, the term “standard rim” is a rim whosespecifications are determined for each tire. For example, the rim is astandard rim in the case of JATMA, the rim is a “Design Rim” in the caseof TRA, and the rim is a “Measuring Rim” in the case of ETRTO. In thespecifications system including specifications on which a tire is based,the term “standard internal pressure” is an air pressure whosespecifications are determined for each tire, and the standard internalpressure is a maximum air pressure in the case of JATMA, the standardinternal pressure is a maximum value described in “TIRE LOAD LIMITS ATVARIOUS COLD INFLATION PRESSURE” in the case of TRA, and the standardinternal pressure is an “INFLATION PRESSURE” in the case of ETRTO. Ifthe tire is for a passenger car, the standard internal pressure is 180kPa.

Further, the “standard internal pressure” is air pressure determined bythe specifications for each tire, and the standard internal pressure ismaximum load ability in the case of the JATMA, and the standard internalpressure is a maximum value described in a table “TIRE LOAD LIMITS ATVARIOUS COLD INFLATION PRESSURES” in the case of the TRA, and the andthe standard internal pressure is “LOAD CAPACITY” in the case of theETRTO.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a tire according to an embodiment of thepresent invention.

FIG. 2 is an enlarged sectional view of a tread portion.

FIG. 3 is a diagram for explaining a rubber thickness between a carcasscord and a belt cord.

FIG. 4 is a diagram showing a standard grounding surface shape of thetire.

FIG. 5 is a diagram showing 80% internal pressure grounding surfaceshape of the tire.

FIG. 6 is a diagram for explaining a problem of a conventionaltechnique.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be explained based on theillustrated example.

FIG. 1 is a sectional view of a heavy load pneumatic radial tire for atruck, a bus or the like according to the present invention, and FIG. 2is an enlarged view of a tread portion of the tire.

In FIG. 1, a pneumatic tire 1 includes a carcass 6 extending from atread portion 2 to a bead core 5 of a bead portion 4 through a sidewallportion 3, and a belt layer 7 disposed inside the tread portion 2 andoutside the carcass 6.

The carcass 6 is formed of one or more (one, in this example) of carcassply 6A in which carcass cords are arranged at an angle of 70 to 90° withrespect to a circumferential direction of the tire. Metal cords such assteel are preferable as the carcass cords, but organic fiber cords suchas nylon, rayon, polyester, aromatic polyamide or the like may also beused if necessary.

The carcass ply 6A includes ply bodies 6 a and 6 a extending between thebead cores 5 and 5, and folded-back portions 6 b and 6 b folded back andretained around the bead cores 5 and 5. A bead apex rubber 8 extendingin a taper manner from the bead core 5 radially outward is disposedbetween the ply body 6 a and the folded-back portion 6 b. The bead apexrubber 8 reinforces a portion of the tire from the bead portion 4 to thesidewall portion 3.

The belt layer 7 is formed of three or more belt plies using metal cordsas the belt cords. In this example, steel cords have four plies, i.e., afirst belt ply 7A which is arranged at an angle of 60±15° with respectto the circumferential direction of the tire and disposed at innermostportion in the radial direction, and second to fourth belt plies 7B, 7Cand 7D arranged at small angle of 10 to 35° with respect to thecircumferential direction of the tire. This belt layer 7 has one or morelocations where the belt cords intersect with each other between theplies. With this structure, the belt rigidity is enhanced, and the treadportion 2 is reinforced with hoop effect.

As shown in FIGS. 2 and 3 in an exaggeration manner, in the tire of thepresent invention, a rubber thickness t1 between the carcass cord 10 ofthe carcass ply 6A and the belt cord 11 of the first belt ply 7A whichis closest to the carcass cord 10 is gradually increased from the tireequator C toward axially outward of the tire. In FIG. 3, the carcasscords 10 and the belt cords 11 have right angle cross section whileignoring the arrangement angle of the cords for convenience's sake.

Especially in this example, an auxiliary rubber layer 12 is interposedbetween the carcass 6 and the belt layer 7 to gradually increase therubber thickness t1. That is, the auxiliary rubber layer 12 includes amain portion 12A which extends from the tire equator C to an outer endof the first belt ply 7A while gradually increasing its thickness. Inthis example, a wing portion 12B which comes into contact with thecarcass 6 and extends and terminates in a buttress portion 3U isconnected to an outer end of the main portion 12A while graduallyreducing a thickness of the wing portion 12B.

A conventional cushion rubber 13 is interposed between the second beltply 7B and the wing portion 12B at outer side of the outer end of thefirst belt ply 7A in the axial direction of the tire, thereby moderatingthe stress concentration at the belt end. As the cushion rubber 13, itis preferable to use a low heating value rubber having small losstangent so that heat is not accumulated. It is also possible to controlthe coating thickness of topping rubber in the carcass ply 6A and/or thefirst belt ply 7A to gradually increasing the rubber thickness t1without providing the auxiliary rubber layer 12.

In a state in which the internal pressure is not charged into the tire,as shown in FIG. 2, the outline Y of the tread portion 2 in themeridional cross section is formed into a double radius shape comprisinga first arc Y1 on the side of the tire equator having a radius ofcurvature R1 whose center is on the tire equator plane, and a second arcY2 on the side of the tread edge having a larger radius of curvature R2than that of the radius of curvature R1 (R2>R1).

With this structure, the tread outline shape in a standard internalpressure state in which the tire is assembled into a standard rim andthe standard internal pressure is charged is brought into closer to asingle arc having a radius of curvature of 650±100 mm.

On the other hand, by bringing the tread outline in the standardinternal pressure is close to the single arc, as shown in FIG. 4, astandard grounding surface shape F0 (so-called foot print) when astandard load is applied to the tire in the standard internal pressureincludes an inner region Fm including a shortest grounding length Lmbetween a central region Fc including a grounding length Lc on the tireequator C at which the grounding length L in the circumferentialdirection of the tire becomes maximum and a tread edge region Feincluding a grounding length Le at the grounding edge TE on the side ofthe tread edge side.

That is, in the standard grounding surface shape F0, the groundinglength Lc on the tire equator C is the longest, the grounding length Leat the grounding edge TE does not become the shortest, and the shortestgrounding length Lm exists between the tire equator C and the groundingedge TE (Lc>Le>Lm). At that time, a distance K1 from the tire equator Cat the position of the shortest grounding length Lm is in a range of 0.5to 0.7 times of a tread half width TW which is a distance from the tireequator C to the grounding edge TE.

With such a structure, a grounding pressure at the tread edge region Feis gradually increased from the inner region Fm toward the groundingedge TE and the grounding is carried out reliably even at the groundingedge TE and thus, the shoulder portion decrease wearing is effectivelyrestrained.

On the contrary, since the grounding pressure is reduced in the innerregion Fm, there is a tendency that new uneven wear is generated in theinner region Fm. Especially in this example, the tread portion 2 isformed with a tread groove 20 which at least includes a circumferentialgroove 20A extending in the circumferential direction of the tirethrough the inner region Fm. Therefore, uneven wear is prone to begenerated inside and outside of the circumferential groove 20A in theaxial direction of the tire. In this example, a circumferential groove20B is further provided in a region which is away from the tire equatorC by a distance K2 which is 0.2±0.05 times of the tread half width TW.

Thereupon, in order to restrain the uneven wear in the inner region Fm,according to the present invention,

1) The rubber thickness t1 between the carcass cord 10 of the carcassply 6A and the belt cord 11 of the first belt ply 7A is graduallyincreased from the tire equator C toward the outside in the axialdirection of the tire, and a rubber thickness t1 q in the half widthposition Q which is away from the tire equator C by a distance K3 whichis 0.55 times of the tread half width TW is set to 0.5 to 3.0 mm;

2) A ratio t2 q/t2 c between the entire thickness t2 q of the treadportion 2 in the half width position Q and the entire thickness t2 c ofthe tread portion 2 in the tire equator C is set to 1.01±0.05.

By employing the above structures 1) and 2), it is possible to enhancethe grounding pressure, and to bring the ratio Lm/Lc between theshortest grounding length Lm and the longest grounding length Lc closerto 0.85 to 0.99 and 1.0 for example. As a result, it is possible torestrain the uneven wear in the inner region Tm, to restrain the unevenwear over the entire width from the tire equator C to the tread edge,and to enhance the wear resistance of the tire.

If the rubber thickness t1 q is less than 0.5 mm, uneven wear isgenerated in the region y away from the tire equator C by a distance of0.5 to 0.7 times of the tread half width TW, and if the rubber thicknesst1q exceeds 3.0 mm, shoulder portion decrease wearing is generated in ashoulder portion. A difference t1 q–t1 c between the rubber thickness t1q and the rubber thickness t1 c on the tire equator C is in a range of0.1 to 1.0 mm.

If the ratio t2 q/t2 c is less than 0.96, uneven wear is generated inthe region y, and if the ratio exceeds 1.06, shoulder wear is generatedin the shoulder portion.

As a result of research of the present inventor, it was found that in anactual running, uneven wear such as shoulder wear was generated in somecases due to difference in running conditions. It is conceived that as amethod for evaluating the wear of a tire, using the standard groundingsurface shape F0, a portion thereof whose grounding length L becomes theshortest causes the wear, but in actual case, a load is applied to rearwheels on an upward slope, or a load is excessively applied to frontwheels on a downward slope or the internal pressure is slightly reducedby size growth of the tire and the tire is deformed largely in manycases and due to this, it is assumed that mismatch with respect to thestandard grounding surface shape F0 is caused.

Thereupon, the present inventor proposed to employ, as a new index forthe wear evaluation, 80% internal pressure grounding surface shape F1when a standard load was applied to a tire in which the tire wasassembled into a rim and 80% internal pressure was charged into thetire. This 80% internal pressure grounding surface shape F1 indicatesthe grounding surface shape when a tire is deformed larger. It was foundthat by using this, it was possible to evaluate an uneven wear over theentire width from the tire equator C to the tread edge in accordancewith actual running, which could not be evaluated by the conventionalmethod.

More specifically, as shown in FIG. 5, in the 80% internal pressuregrounding surface shape F1, a ratio Lsm/Lsc between the shortestgrounding length Lsm in a region Z from a position away from the tireequator C by a distance K4 which is 0.5 times of the tread half width TWand the grounding length Lsc at the tire equator C is set in a range of0.75 to 0.99. If the ratio is in this range, it is possible to restrainthe uneven wear at a position of the shortest grounding length Lsm, andto enhance the uneven wear over the entire width from the tire equator Cto the tread edge.

If the ratio Lsm/Lsc is less than 0.75, uneven wear is generated in aposition near the shortest grounding length Lsm, and if the ratioexceeds 0.99, uneven wear is generated in a position near the tireequator. As described above, adverse effects are invited.

Although the preferred embodiment of the present invention has beendescribed in detail, the invention is not limited to the illustratedembodiment, and can also be applied to tires in various categories suchas a passenger car, a small truck, a construction or industrial vehicleand the like.

Embodiment

Heavy load tires of tire size of 295/80R22.5 having the structure shownin FIG. 1 were prototyped based on the specification shown in Table 1,and uneven wear performances of the prototyped tires were tested.Results thereof are shown in Table 1. Here, in “relation of groundinglength L” in Table 1, cases in which the grounding length Lc on the tireequator C is the longest, the grounding length Le at the grounding edgeTE is not the shortest, and the shortest grounding length Lm existsbetween the tire equator C and the grounding edge TE (Lc>Le>Lm) areshown with “0”, and other cases such as a case in which the groundinglength Le at the grounding edge TE is the shortest are shown with “x”.

(1) Uneven Wear Performance;

The prototyped tires were mounted to all wheels of a truck (2-2/D type)with rims (22.5×8.25) and internal pressure (900 kPa), the vehicle wasallowed to run the distance of 50,000 km, and amounts of groove depthsreduced in the circumferential grooves 20A and 20B caused by wear afterrunning were compared with each other. If the reduced amount is large, adifference in wear is large and uneven wear performance is inferior.

TABLE 1 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5Tread radius of curvature <mm> in standard internal pressure state 640640 640 570 730 Rubber thickness between cords of carcass and belt layert1q <mm> 0.8 1.5 2.4 1.6 1.6 t1c <mm> 0.5 0.5 0.5 0.5 0.5 Entirethickness of tread t2q <mm> 31.8 32.0 32.6 32.1 32.1 t2c <mm> 32.0 32.032.0 32.0 32.0 Ratio t2q/t2c 1.0 1.01 1.03 1.01 1.01 Standard groundingsurface shape FIG. 4 FIG. 4 FIG. 4 FIG. 4 FIG. 4 Relation of groundinglength L O O O O O Ratio Lm/Lc 0.85 0.96 0.98 0.96 0.96 0.8 internalpressure grounding surface shape FIG. 5 FIG. 5 FIG. 5 FIG. 5 FIG. 5Ratio Lsm/Lsc 0.75 0.92 0.95 0.92 0.92 Uneven wear performance Reducedamount <mm> of circumferential groove 20B 4.7 4.9 4.7 4.7 5.1 Reducedamount <mm> of circumferential groove 20A 5.9 5.1 4.8 5.3 3.9 DifferenceB − A <mm> 1.2 0.2 0.1 0.6 −1.2 Comparative Comparative ComparativeComparative example 1 example 2 example 3 example 4 Tread radius ofcurvature <mm> in standard internal pressure state 520 640 645 800Rubber thickness between cords of carcass and belt layer t1q <mm> 0.20.3 3.1 3.2 t1c <mm> 0.5 0.5 0.5 0.5 Entire thickness of tread t2q <mm>32.1 31.3 34.1 34.2 t2c <mm> 32.0 32.0 32.0 32.0 Ratio t2q/t2c 0.95 1.01.0 1.08 Standard grounding surface shape — FIG. 4 FIG. 4 FIG. 4Relation of grounding length L × (Le is O O O shortest) Ratio Lm/Lc 0.800.79 1.01 1.02 0.8 internal pressure grounding surface shape — FIG. 5FIG. 5 FIG. 5 Ratio Lsm/Lsc 0.7 0.7 1.01 1.04 Uneven wear performanceReduced amount <mm> of circumferential groove 20B 4.2 4.5 7.4 7.6Reduced amount <mm> of circumferential groove 20A 6.4 6.8 5.1 4.9Difference B − A <mm> 2.2 2.3 −2.3 −2.7

As shown in the Table, it can be confirmed that in the case of the tiresof the embodiments, the uneven wear is enhanced.

INDUSTRIAL APPLICABILITY

According to the pneumatic tire of the present invention as describedabove, it is possible to restrain the uneven wear from the tire equatorto the tread edge, to enhance the wear resistance, and to suitably applythe invention to a heavy load tire used under high internal pressure andhigh load.

1. A pneumatic tire comprising a carcass including a carcass plyextending from a tread portion to a bead core of a bead portion througha sidewall portion, and a belt layer having a belt ply arranged insidethe tread portion and outside the carcass, wherein a standard groundingsurface shape when a standard load is applied to the pneumatic tire in astate in which the tire is assembled into a standard rim and a standardinternal pressure is charged into the tire includes an inner regionincluding a shortest grounding length Lm between a central regionincluding a grounding length Lc at a tire equator in which a groundinglength L in a circumferential direction of the tire becomes maximum anda tread edge region including a ground length Le at a grounding edge,said grounding length Lm the shorter than the grounding length Lc andthe grounding length Le, the shortest grounding length Lm is located ina range apart from the tire equator by a distance of 0.5 to 0.9 times ofa tread half width TW, a rubber thickness t1 between a carcass cord ofthe carcass ply and a belt cord of the radially inward belt ply which isthe closest to the carcass cord is gradually increased outward in anaxial direction of the tire from the tire equator, furthermore, therubber thickness t1 q in a half width position of 0.55 times of thetread half width TW from the tire equator is set to 0.5 to 3.0mm,additionally, a ratio t2 q/t2 c between an entire thickness t2 q of thetread portion if the half width position and an entire thickness t2 c ofthe tread portion in the tire equator is set to 1.01±0.05.
 2. Apneumatic tire according to claim 1, wherein in 80% internal pressuregrounding surface shape when a standard load is applied to the tire 80%internal pressure state in which the tire is assembled into the standardrim and 80% internal pressure of the standard internal pressure ischarged, a ratio Lsm/Lsc between a grounding length Lsm which becomesthe shortest in a region from a position away from the tire equator by adistance of 0.5 times of the tread half width TW to the grounding edgeand a grounding length Lsc at the tire equator is set to 0.75 to 0.99.3. A pneumatic tire according to claim 1 or 2, wherein the belt cord ofthe belt layer is a metal cord.
 4. A pneumatic tire according to claim1, wherein the belt layer comprises three or more layers, and thepnematic tire is a heavy load radial tire.
 5. A pneumatic tire accordingto claim 1, wherein the inner region is provided with a circumferentialgroove extending in the circumferential direction of the tire.
 6. Apneumatic tire according to claim 5, wherein the circumferential grooveis provided in a region away from the tire equator by a distance of0.2±0.05 times of the tread half width TW.